A conventional moldset for plastic injection molding is constructed of high-strength metals and most specifically tool steels having a compressive yield strength at 0.5% elongation exceeding 100,000 psi. The mold cavity itself is defined by a moldset parting line which opens and closes with each mold cycle, and on each side of the parting line is at least one part-forming mold insert surface, which in turn, is supported by these suitably-rigid moldset members so that very high force must be exerted to produce even ten millionths of an inch in deflected distance relative to the mold clamp plate, when distances are measured at constant temperature. In this manner, a rigid moldset is used to form a closed mold cavity of comparatively fixed volume. From this mold is produced a molded part which must meet a product specification for thickness, with usual variation of at least + or -0.002" being acceptable (commonly, far more).
For conventional injection molding, such a conventional moldset is quite satisfactory, as long as sufficient mold-clamping force is exerted by the injection molding machine to overcome the pressure exerted on the part-forming surfaces by the injected plastic melt, and at least enough injected plastic melt volume to at least initially fill the closed mold cavity.
A "short shot" would be defined as an injected melt volume insufficient to at least fill out the cavity configuration, such that the end-of-flow perimeter of the molded part is visibly misshapen. "Flash" is defined as an excess of injected melt volume such that the mold-clamping force has been exceeded by the injected melt's pressure on the mold, so that plastic melt leakage into the parting line has visually occurred. By ordinary standards, a molder is said to have completed his job satisfactorily as long as he maintains his processing conditions somewhere between these "short shot" and "flash" condition boundaries.
However, with the continuing development of improved engineering plastics having much greater dimensional stability at elevated temperatures and under load (i.e., creep resistance), plastic has increasingly able to be competitive in very-high-precision manufactured parts which otherwise would have been made out of a precision machined metal or lapped glass. Such an example is computer hard disk drive media, wherein the disk substrate has been traditionally aluminum which is machined and lapped to extremely high tolerances and is now under competition from optical glass flats which are suitably lapped for even better micro-surface and planarity. Another example is in optical lenses which traditionally have been ground and polished glass.
These examples are part of a class of potential applications for molded plastic in competition to machined and/or lapped-metal or glass parts. The product tolerances in each case permit some variation in thickness from nominal (here, nominal=that central value, around which there is some defined range of also acceptable greater and lesser values, which are the "plus or minus" part of specification values) but are quite strict regarding imperfect surface quality or imprecise surface contour. For example, camera optical lenses must be true to their nominal radius of curvature within less than one wavelength of light and are typically checked by interferometric measurements for accuracy of surface contour. Surface RMS value is also measured in a fractional microns. In contrast, the acceptable variation from nominal thickness is at least + or -0.002 inch, orders of magnitude larger variation than is tolerable for surface contour deviation or roughness.
Similar tolerances exist for optical disk and even more stringent surface tolerance for magnetic-media hard disk substrate.
In each case, this higher priority for surface accuracy and macro-texture should dictate a correspondingly high concern over those factors which in plastic molding dictate the quality of molded-part-surface replication of the extremely-high-precision mold surfaces against which the molded plastic part is formed. However, even perfection in the mold surface does not by any means guarantee perfection in the corresponding plastic part made therefrom, since the degree of surface replication quality is well known to correlate extremely strongly with the mold packing parameter. Most specifically, the plastic must not be allowed to shrink away from the precision mold surface before the plastic has sufficiently cooled to fully replicate the surface detail and contour of the mold surface, and this can only be assured by putting the plastic melt under continuing pressure and this enforced densification compensates for the inevitable degree of thermal shrinkage of the plastic during its cooling process.
In recognition of this strong predictive correlation between mold packing and corresponding melt density and pressures, and resulting mold-part accuracy of surface contour and micro-finish, a greater need for monitoring and/or controlling the melt density and pressures within the filled mold cavity have become correspondingly important to this field of the present invention, since controlling these undesirable vatiations is much more difficult in multicavity molding vs. single cavity molding.
Johnson (U.S. Pat. No. 2,443,826 issued Jun. 22, 1948) teaches a lens molding apparatus with coiled steel compression springs whose function is to become compressed as the injected melt's pressure rises to its peak, then as cooling and shrinkage starts, the spring recovers and exerts pressure upon the plastic. It uses this apparatus to "form lenses of predetermined thickness", which is achieved by intentionally fully deflecting the dies (his (12) and (13)) till they reach his "stops {which} in this case are simply the annular bases of the sockets 14 and 15", which causes these relatively weak coil springs (his (17) and (18)) to be fully compressed. See column 2, lines 35-48.
However, there is no apparent concern for nor provision for preventing the spring from "bottoming out" (loaded so as to allow the rigid elements of the spring-loaded assembly to rest against each other, so that no further compressive displacement is possible regardless of the additional load). Once "bottomed out", the melt pressure is free to vary uncontrollably (which results in unpredictable mold packing, melt density variations, and corresponding variations in molded part's surface contour and microfinish), up to the point where the melt pressure is so high it overcomes the clamping force and flashes the mold. Nor does Johnson's apparatus recognize any value to any stiffer types of steel springs (since he is apparently unaware of any disadvantage to allowing his springs to bottom out), nor any alternatives to conventional metal springs at all, such as are taught by the present invention.
Furthermore, Johnson doesn't teach any means for measuring cavity melt pressure, spring load or deflection, nor any of the molded part's surface quality attributes. Never mentioned is any value in monitoring or correcting cavity imbalance nor compensating to prevent flash in a multicavity moldset, even though Johnson's illustration shows a 2-cavity mold. Nor does Johnson speak to the need to measure or control cavity melt pressure in order to assure the molded part's surface contour or microfinish.
The most primitive attempts achieve the latter objectives date back to the 1960's, by simply implanting a strain-gauge-type pressure transducer into a conventional moldset which forms a fixed volumetric cavity with rigidly supported part-forming mold inserts. This pressure transducer is placed in communication with the plastic melt at some point within the moldset, either in direct contact with the melt itself (i.e., implanted inside the mold cavity or in the runner and/or gate area leading into the mold cavity), or else to place such a transducer behind a movable tool steel pin or rod, which in turn is pressurized by its contact with the plastic melt (such as is the case for an ejector pin, placed in contact with the plastic runner and/or knockout tabs at the end-of-fill position on the molded plastic part itself).
An alternative method of monitoring in indirect fashion the cavity melt pressure is to implant at the parting line of the conventional moldset described above a proximity sensor, which resolves to millionths of an inch, to measure the tendency of A and B mold plates to separate at the parting line as the molding process cycle sequence moves from an empty mold to a filled mold to packing pressure at maximum and then dimishes as cooling and shrinkage take place. Its advantages are:
1. it is no longer a one-point measurement within the surfaces wetted by the plastic melt PA1 2. it is comparatively easy to install without redesign of the mold PA1 1. The proximity-sensed micro-gap at the parting line due to the melt pressure can be exceeded by an order of magnitude by the changes in temperature which occur from a mold at room temperature as compared to the same mold at its elevated operating temperatures. In other words, the signal-to-noise ratio is potentially very suspect unless very good compensation is made for these thermal expansion characteristics of the mold. PA1 2. In a multi-cavity moldset, the parting line micro-gap represents the sum of the individual cavity melt pressures exerted against the mold clamping force, and there is no way to determine thereby such forces acting in individual cavities. Most specifically, there is no insight given into the possible imbalance which can exist from an individual cavity to another individual cavity, and since such information is not available, it is not possible thereby to use this generally predictive technique for problem-solving in correcting for cavity imbalance. PA1 (1) European patent application No. 0128722, on their application #84303756.5, filed Jun. 6, 1984 PA1 (2) European patent application No. 0130769, on their application #84304290.4, filed Jun. 25, 1984. PA1 1. On either the stationary or movable halves of the moldset, there must be a resilient member interposed between the part-forming mold insert surface and the associated clamp plate for that half of the moldset. Yet, like a conventional moldset, this "adaptive" mold's clamping force is still transmitted through the mold clamp plate to the mold insert, and conversely, force transmitted by the cavity's contents--the molten plastic's exerted pressure --pushing internally against the opposing mold inserts' partforming surface, is further transmitted back to the corresponding mold clamp plate. This path of mechanical support and force transmission must contain a resilient member, preferably such as a steel mechanical spring but also could be an elastomeric polymer of known and predetermined modulus, or a hydraulic or pneumatic cylinder. PA1 2. When the mold is initially closed but not filled, there exists a first position having a first separation distance between the stationary-half and movable-half mold inserts, and this first separation distance is equal to or less than the molded product's specification value for minimum acceptable part thickness. PA1 3. At least sufficient volume of injected plastic melt must enter the adaptive mold cavity to cause at least some deflection of the resilient member toward its corresponding clamp plate, to assure that the resulting molded-part thickness will be somewhere between minimum and maximum thickness tolerances. This second position of the opposing mold inserts corresponds to a second separation distance which must also be less than or equal to the maximum acceptable molded-part thickness under the product specification. At maximum acceptable volume of the injected melt, the resilient member is still incompletely compressed and doesn't bottom out. PA1 4. Within that minimum and maximum thickness range, variations in melt volume or packing pressure will cause changes in the deflected-cavity positional displacement and correspondingly in cavity volume, (thus resulting in changed molded-part thickness and mass). Yet, in spite of cycle-to-cycle variations (or even cavity-to-cavity variations in multicavity molding) in the molding process, the resulting melt density and pressure during packing and cooling stages of the molding cycle will stay inside the acceptable range. PA1 a. the injected melt pushes upon the adaptive cavity's partforming surface PA1 b. the resilient member is partially compressed, within the previously-defined range of deflection PA1 c. each increment of deflection increases the volume occupied by the injected melt and correspondingly drops its melt pressure, while increasing incrementally the opposing "spring" force (exception: certain hydraulic cylinder embodiments shown later) PA1 d. as a result, the applied mold packing force exerted by the melt declines until it now equals the opposing resilient member's increased "spring" force PA1 e. at which point the rearward deflection "stalls out" (without having "bottomed out" against a mechanical hardstop) PA1 f. as time passes and the melt cools, the resulting volumetric shrinkage causes the resilient member to recover gradually part (but not all) of its uncompressed length
This parting line micro-gap distance is proportional to an average force exerted by the melt, which is striving to drive apart these mold plates, which are held together by the clamping force of the molding machine. This technology has been commercialized recently by K-Tron, a subsidiary of Kodak. See 1988 SPE ANTEC paper reference. However, even this new indirect method of cavity-melt-pressure and density prediction still has limitations:
Furthermore, if an erroneous setup allows more than a slight excess of plastic to be injected, the resulting overfill causes flashing to occur, which can cause mold damage if undetected.
Another interesting technique monitors the melt pressure in an anticipatory fashion before the mold cavity is fully filled, by means of measuring upstream within the runner system the instantaneous changes in melt pressure, which would then plot out as a melt-pressure waveform versus time. This observed waveform is compared to a reference waveform, to give a predetermined change in molding machine setup, specifically the change in clamping force applied to the moldset to compress and further densify the injected plastic melt within the mold cavity. These are the Technoplas references:
However, it doesn't help monitor and correct for cavity-to-cavity imbalance in a multicavity moldset, nor protect against flashing if more than slight overfill were to happen. Also, these teachings of Technoplas clearly are limited to single-cavity molding in that, even though individual cavity melt-pressure waveforms could be sensed, only one setting for the machine clamp can be chosen, which will then act equally upon all.